This invention relates to a fuel injecting device for supplying fuel to a Diesel engine, more specifically to a fuel injecting device for an internal combustion engine including a so-called unit injector in which an injection valve for each cylinder is integral with a pump section for high-pressure fuel supply.
In order to improve the combustion efficiency of a Diesel engine, it is necessary to increase fuel injection pressure. A conventional injecting device generally uses an injection pressure of approximately 700 atms. However, higher injection pressure is required for improved combustion efficiency, and the conventional device cannot meet this requirement.
For the high-pressure injection, there is proposed the so-called unit injector in which an injection valve is integral with a pump section for supplying high-pressure fuel to the valve.
In a unit injector of this type disclosed in USP No. 4,235,374, a primary pumping plunger driven by an engine and a secondary plunger for delivering high-pressure fuel to a nozzle are fitted in a unit injector housing for reciprocation. A timing chamber is defined between the primary and secondary plungers, while a metering chamber is defined between the secondary plunger and the nozzle. The timing chamber is supplied with fuel through a conduit, which is opened and closed by a control valve for determining the injection timing.
In this prior art device, high pressure in the timing chamber is directly applied to the control valve till the injection is finished after the conduit is closed by the control valve. Thus, the prior art device is subjected to many problems regarding the durability and working accuracy of the control valve.
Immediately after the conduit is closed, the pressure inside the timing chamber is suddenly increased and applied to the control valve. To protect the control valve against such high pressure, an orifice may be provided between the control valve and the timing chamber. In this case, however, fuel for the next process should be supplied to the timing chamber through the orifice when the primary plunger is raised. It is therefore impossible to make quick fuel supply.